1. Field of the Invention
The present invention relates to an optical disc device and media type determination method, and is preferably applied to an optical disc device that supports a variety of optical discs, for example.
2. Description of the Related Art
Optical disc devices have been popular: such optical disc devices emit an optical beam to a recording medium, or an optical disc, in order to either record information on it or reproduce information from it.
Among those optical disc devices, there is the one that supports both Compact Disc (CD), for which an optical beam with a wavelength of about 780 nm is used, and Digital Versatile Disc (DVD), for which an optical beam with a wavelength of about 660 nm is used.
These discoid discs, CD and DVD, have the same outside diameter and thickness, about 120 mm and 1.2 mm respectively. What is different between them is the thickness of a cover layer, i.e. the distance from the surface of a disc to its signal recording surface where information is recorded: CD's is approximately 1.2 mm while DVD's is about 0.6 mm.
Moreover, the numerical aperture (NA) of an optical disc device's objective lens, through which an optical beam is emitted to an optical disc, is predetermined: the numerical apertures are 0.45 for CD and 0.6 for DVD.
Among the optical disc devices, there is the one that supports CD and DVD by using an two-focal-points objective lens having two focal points.
When an optical disc is put in it, the optical disc device, for example, recognizes the distance from the surface of the optical disc to its signal recording surface by detecting a reflection optical beam, which is the reflection of an optical beam emitted to the optical disc, makes a determination as to whether the optical disc is CD or DVD according to the distance, and appropriately focuses the optical beam on the signal recording surface. At this time, the optical disc device eliminates the elements of the reflection optical beam that are obtained from a false focal point, which is not the desired one of all the focal points (see Jpn. Pat. Laid-open Publication No. 2003-157545 [FIG. 2], for example).